Example CF-NetCDF for Satellite

Back to WCS Access to netCDF Files

A Real life example how to download and store satellite data.

Total Ozone Mapping Spectrometer (TOMS) satellite

This data has been collected by three different satellites from 1979 to present. It is downloadable in text files, linear rectangular 288 * 180 grid.

Nimbus satellite:

• Data download site. ftp://toms.gsfc.nasa.gov/pub/omi/data/aerosol/
• First 1978-11-01: ftp://toms.gsfc.nasa.gov/pub/nimbus7/data/aerosol/Y1978/L3_aersl_n7t_19781101.txt
• Last 1993-05-06: ftp://toms.gsfc.nasa.gov/pub/nimbus7/data/aerosol/Y1993/L3_aersl_n7t_19930506.txt

EPTOMS satellite:

• Data download site ftp://toms.gsfc.nasa.gov/pub/eptoms/data/aerosol/
• First 1996-07-22: ftp://toms.gsfc.nasa.gov/pub/eptoms/data/aerosol/Y1996/L3_aersl_ept_19960722.txt
• Last 2007-12-31: ftp://toms.gsfc.nasa.gov/pub/eptoms/data/aerosol/Y2005/L3_aersl_ept_20051231.txt

OMI satellite:

• data download site: ftp://toms.gsfc.nasa.gov/pub/omi/data/aerosol/
• First 2004-10-01: ftp://toms.gsfc.nasa.gov/pub/omi/data/aerosol/Y2004/L3_aersl_omi_20041001.txt
• Year 2011, growing: ftp://toms.gsfc.nasa.gov/pub/omi/data/aerosol/Y2011

As you can see, there is a gap between 1993-05-06 and 1996-07-22.

The python module AI_data.py contains the templates for ftp data urls, like

   template_path_omi = '/pub/omi/data/aerosol/Y%Y/L3_aersl_omi_%Y%m%d.txt'
   first_omi_datetime = datetime.datetime(2004, 9, 6)

The %Y %m %d are python format codes, 4-digit year, 2-digit month, 2-digit day.

a programmer can now get the url:

   AI_data.determine_ftp_path(datetime.datetime(2010, 3, 24)

returns '/pub/omi/data/aerosol/Y%Y/L3_aersl_omi_20100324.txt'

Creating an Empty CF-NetCDF File

NetCDF Markup Language (NCML)

NCML documentation

We describe AerosolIndex.nc CF-NetCDF file in xml file AerosolIndex.ncml

The first line is the root element and namespace

   <netcdf xmlns="http://www.unidata.ucar.edu/namespaces/netcdf/ncml-2.2">

The explicit means, that all the metadata is here. Alternatively, readMetadata would require an existing source netcdf file.

       <explicit />

Global attributes. Notice, that NCML does not require CF conventions. Therefore, you have to declare the convention yourself.

       <attribute name="title" type="string" value="NASA TOMS Project" />
       <attribute name="comment" type="string" value="NASA Total Ozone Mapping Spectrometer Project" />
       <attribute name="Conventions" type="string" value="CF-1.0" />

Declare dimensions. This is a 3-dimensional grid, with time as the unlimited dimension.

       <dimension name="time" length="0" isUnlimited="true" />
       <dimension name="lat" length="180" />
       <dimension name="lon" length="288" />

       <variable name="time" type="int" shape="time">
           <attribute name="standard_name" type="string" value="time" />
           <attribute name="long_name" type="string" value="time" />
           <attribute name="units" type="string" value="days since 1979-01-01" />
           <attribute name="axis" type="string" value="T" />
       </variable>
       <variable name="lat" type="double" shape="lat">
           <attribute name="standard_name" type="string" value="latitude" />
           <attribute name="long_name" type="string" value="latitude" />
           <attribute name="units" type="string" value="degrees_north" />
           <attribute name="axis" type="string" value="Y" />
           <values start="-89.5" increment="1" />
       </variable>
       <variable name="lon" type="double" shape="lon">
           <attribute name="standard_name" type="string" value="longitude" />
           <attribute name="long_name" type="string" value="longitude" />
           <attribute name="units" type="string" value="degrees_east" />
           <attribute name="axis" type="string" value="X" />
           <values start="-179.375" increment="1.25" />
       </variable>
       <variable name="AI" type="float" shape="time lat lon">
           <attribute name="long_name" type="string" value="Aerosol Index" />
           <attribute name="units" type="string" value="fraction" />
           <attribute name="_FillValue" type="float" value="NaN" />
           <attribute name="missing_value" type="float" value="NaN" />
       </variable>
   </netcdf>

Create Script AI_create.py

   from datafed import cf1
   
   def create():
       cf1.create_ncml22('AerosolIndex.nc', 'AerosolIndex.ncml', '64bitoffset')
   
   if __name__ == "__main__":
       create()